Surface modified hollow microspheres

ABSTRACT

Non-fused hollow glass microspheres that consist primarily of silicate are rendered more water-resistant by treatment with multivalent metals. The hollow microspheres are slurried with water and then a solution of a multivalent metal salt such as Al 2  (SO 4 ) 3  is added to the slurry. The rate of the addition is controlled so that the pH can be regulated. Separating, washing and drying complete the preparation.

BACKGROUND OF THE INVENTION

This invention relates to the treatment of silicate based hollowmicrospheres to improve their stability in aqueous environments. Theinvention comprises treating said microspheres with multivalent metalions, thereby modifying the surface of the microspheres.

Hollow microspheres such as those described in U.S. Pat. Nos. 3,796,777;3,794,503 and 3,888,957 are formed by spray drying alkali metal silicatesolutions that have been combined with "polysalt" solutions. While theseproducts are satisfactory for use in essentially non-aqueousenvironments, they are sufficiently sensitive to water to prevent theiruniversal acceptance in aqueous systems.

Fused glasses, used in bottles and similar items, have been treated withmultivalent metal ions to improve weathering qualities of the product.Such treatment consists of the vapor phase reaction of a compound suchas aluminum chloride with the glass as it is cooling from fabricatingtemperatures of up to 1000° C. During this time, glass is considered tobe in its most reactive state. U.S. Pat. No. 3,337,321 and Swedish Pat.No. 10372/1955 (issued May 4, 1961) illustrate such processes.

SUMMARY OF THE INVENTION

The water resistance of non-fused hollow microspheres consisting largelyof silicate can be improved by modifying their surfaces with suitablemultivalent metal ions. Such modification is achieved by addingsolutions containing appropriate metal salts such as AlCl₃ or Al₂ (SO₄)₃to aqueous slurries of the microspheres while controlling the pH. Themicrospheres recovered from the slurry and then dried are found to bewater resistant and are useful in aqueous environments.

THE INVENTION

Hollow microspheres primarily comprising silicate that have not beenfused or sintered require surface modification to provide stableproperties on exposure to aqueous environments. While many materialsbenefit from the process of our invention, we prefer to use the hollowmicrospheres described in U.S. Pat. Nos. 3,794,503; 3,796,777 and3,880,957. These patents are hereby incorporated by reference asdescribing the preferred hollow microspheres and the method of preparingthem in detail.

Hollow microspheres are prepared by spray drying a homogeneous solutionof sodium silicate and a "polysalt." The product from the spray dryerhas a particle diameter of 35 to 150 microns (100 to 400 mesh) and lessthan 25% moisture. This product is heated and the resulting microsphereshave diameters of about 35 to 2000 microns (10 to 400 mesh), bulkdensities of 2 to 20 lbs/cu. ft. and less than 7% moisture. The ratio ofpolysalt solids to sodium silicate solids is 0.02:1 to 2.0:1 and thesodium silicate contains 1.5 to 4.0 moles of SiO₂ per mole of Na₂ O.U.S. Pat. No. 3,794,503 describes "polysalts" as salts with anion tocation ratios that are reduced when the salts dissolve and becomehydrolyzed. Ammonium pentaborate, sodium pentaborate and sodiumhexametaphosphate are preferred "polysalts," of which ammoniumpentaborate is the most preferred.

Either the intermediate product from the spray dryer or the product fromthe final drying step can be processed according to our process. Themicrospheres are slurried with water and agitated while a solutioncontaining the multivalent ion is added slowly to said slurry. The pH ofthe slurry is controlled by varying the rate at which the solution isadded. After the solution has been added and the ion exchange completed,the microspheres that remain floating are separated from the liquid anddried, thereby recovering the improved product. The microspheres may bewashed before drying.

Any multivalent metal that achieves the desired decrease in solubilityof the microspheres is useful in the process of our invention. Examplesinclude among others calcium, magnesium, aluminum and zinc. Such metalsare used as the salts of strong acids such as chloride, nitrates,sulfates and the like. The salts are dissolved in any manner required toform a stable solution at a somewhat acid pH. The amount of metal ionrequired depends on the composition of the microspheres and the degreeof property modification desired. In general, 1 part by weight (pbw) isrequired for each 2 to 20 pbw of microspheres.

The microspheres either from the spray dryer or the second heating stepmust be slurried with sufficient water to prevent shearing stress andconsequent fractures of the microspheres. We find that about 5 to 20% byweight of hollow microspheres is satisfactory. Stirring must be adequateto ensure good mixing when the metal salt solution is added. The rate atwhich said metal salt solution is added must be controlled to provide apH in the range which promotes the yield of intact spheres with thedesired surface modification. Surprisingly, this pH range must be variedaccording to the silicate used in preparing the microspheres. If thesilicate used had up to about 2.4 moles of SiO₂ per mole of Na₂ O the pHmust be maintained above about 5 with the preferred range of 5.0 to 7.0.If the silicate had about 2.4 to 4.0 moles of SiO₂ per mole of Na₂ O thepH can be as low as 4 with a preferred range of 4.0 to 6.5. Once all ofthe metal salt solution has been added and the slurry is at the correctpH it can be stirred for an additional period of up to an hour toprovide time for the surface reaction to be completed.

A preferred embodiment of the process of our invention is carried out asfollows. The sodium silicate and "polysalt" solutions are combined andthen spray dried to provide hollow microspheres of 18 to 25% moistureand a particle size of 35 to 150 microns. This material is slurried withsufficient water to allow stirring without fracturing the microspheres.Then the metal salt solution is added at a rate which controls the pH inthe required range. Additional pH buffers and adjusting agents may beadded to provide the correct pH. After the reaction is complete thefloating microspheres are separated from the slurry, washed, dried andexpanded by heating, thereby providing the desired product.

The drying and expanding involves heating the microspheres to about 300°to 450° C. over a period of several hours in successive steps.

Our product, a surface modified hollow microsphere, has a particlediameter of about 35 to 2000 microns, a bulk density of 2 to 20lb/cu.ft. and less than 7% moisture. The composition of the microspherenow includes from 0.1 to about 5.0% of the multivalent metal. The exactmechanism by which the multivalent metal is incorporated into themicrosphere is not fully understood, but it appears that it does replacesome of the alkali metal from said composition.

The water resistance imparted to the hollow microspheres is measured byslurrying the product in water, determining the percentage of floatingmaterial after 1 hour and after 24 hours. Comparison of the two resultsis a measure of the insoluble character of the material and, of course,the smaller the difference between the microspheres floating after 1hour and after 24 hours the greater the water resistant character.

The product of our process is useful in any of the usual applications ofhollow microspheres, and especially useful in aqueous systems.

EXAMPLES

The following examples illustrate certain embodiments of the process ofour invention. The purpose of these examples does not includeestablishing the scope of the invention which is defined in thedisclosure and claims.

Our test for water resistance was carried out as follows. Two grams ofthe microspheres were placed in 200 ml of deionized water in anErlenmeyer flask. The flask is shaken continuously using a wrist shaker.The percentage of floating spheres is determined after 1 hour and after24 hours, and the difference in the percentages reported. The floatingmaterial is measured by allowing the slurry to sit until the solid andliquid phases separate, usually about 1 hour. The floating hollowmicrospheres are skimmed from the surface of the water and washed into atared sample cup. Any remaining floating spheres adhere to the edge of aweighed filter paper which is placed in the sample cup. The samples aredried at 103° C. for 24 hours before final weighing.

EXAMPLE 1

This example illustrates the process of our invention employed inimproving the water resistant character of a hollow microsphere madewith sodium silicate having 2.0 moles of SiO₂ per mole of Na₂ O. Themicrospheres were prepared as described in Example 1 of U.S. Pat. No.3,796,777. The final product had the following properties:

    ______________________________________                                        SiO.sub.2            56%                                                      Na.sub.2 O           28%                                                      B.sub.2 O.sub.3      12.7%                                                    H.sub.2 O            3.3%                                                     True Density         12 lb/cu.ft.                                             ______________________________________                                    

Three slurries containing 15% of these microspheres in water wereprepared and stirred. Solutions containing 12% of MgCl₂, Ca(NO₃)₂ andAl₂ (SO₄)₃ were added slowly to these slurries so that the pH of thematerial did not go below 5.0. In each case, sufficient slurry was addedto provide 1 pbw of the metal for each 5 pbw of the microspheres. Afterall of the metal solution was added, the slurries were stirred for anadditional 15 minutes. The floating spheres were separated, washed anddried. The microspheres were dried by heating at 105° C. for 18 hours.

The product microspheres and the raw material were tested as previouslydescribed and the results are summarized in the following table.

    ______________________________________                                                  Floaters    Floaters  Change                                        Treatment 1 hr. (%)   24 hrs. (%)                                                                             (%)                                           ______________________________________                                        None      61.2        10.5      50.7                                          MgCl.sub.2                                                                              82.0        68.3      13.7                                          Ca(NO.sub.3).sub.2                                                                      78.8        66.8      12.0                                          Al.sub.2 (SO.sub.4).sub.3                                                               89.0        83.0       6.0                                          ______________________________________                                    

These results definitely show the improvement desired in the treatedmicrospheres with the aluminum treatment appearing to be the best.

EXAMPLE 2

The process described in Example 1 was repeated except that theintermediate spray-dryer product was used. This material had thefollowing properties:

    ______________________________________                                                SiO.sub.2    50.8%                                                            Na.sub.2 O   25.4                                                             B.sub.2 O.sub.3                                                                            11.6                                                             H.sub.2 O    12.2                                                     ______________________________________                                    

All reaction conditions were the same except that the drying step was asfollows: 24 hours at about 100° C., 1 hour at 150° C., 1 hour at 200° C.and 3 hours at 300° C. The results were comparable to those reported inExample 1.

EXAMPLE 3

This example illustrates the preparation of an improved product by theprocess of our invention carried out on a microsphere prepared with asodium silicate with 2.4 moles of SiO₂ per mole of Na₂ O. Thesemicrospheres had the following properties:

    ______________________________________                                                SiO.sub.2    59.4%                                                            Na.sub.2 O   23.1%                                                            B.sub.2 O    10.2%                                                            H.sub.2 O     5.3%                                                    ______________________________________                                    

A slurry containing 12% of the microspheres in water was stirred whileslowly adding a 0.45 M solution of Al₂ (SO₄)₃ 18H₂ O. The rate ofaddition was controlled to maintain the pH at 4.0 or above andsufficient of the solution was added to provide 3.33 pbw of microspheresfor each pbw of the salt. After the addition was completed, stirringcontinued for 15 minutes during which time the pH rose to 5.5. Thefloating microspheres were separated, washed and dried. Drying included18 hours at 105°-115° C., an hour each at 150° C. and 200° C., and 3hours at 300° C.

The product microspheres and the raw material microspheres were testedas previously described and the results are summarized in the followingtable:

    ______________________________________                                                    Floaters   Floaters  Change                                       Treatment   1 hr. (%)  24 hrs. (%)                                                                             (%)                                          ______________________________________                                        None        72.5       59.6      13.1                                         Al.sub.2 (SO.sub.z).sub.3 3H.sub.2 O                                                      80.0       76.2       3.8                                         ______________________________________                                    

These results show the improvement desired.

We claim:
 1. Water resistant hollow microspheres that have not beenexposed to fusion or sintering temperatures, said microspheres beingcharacterized by a weight % ratio of "polysalt" solids to sodiumsilicate solids of 0.02:1 to 2.0:1, said polysalt being a colloidalinorganic salt of an alkali metal and/or ammonium cations wherein theanion to cation ratio is reduced when the salt is dissolved and becomeshydrolized and said sodium silicate containing 1.5 to 4.0 moles of SiO₂per mole of Na₂ O; a particle size of about 35 to 2000 microns; a bulkdensity of 2 to 20 lbs/cu ft, less than 7% moisture and 0.1 to 5.0% of amultivalent metal that decreases the solubility of the microsphere inwater.
 2. The hollow microsphere of claim 1 wherein the multivalentmetal is calcium, magnesium, aluminum or zinc.
 3. The microsphere ofeither of claims 1 or 2 wherein said polysalt is ammonium pentaborate,sodium pentaborate or sodium hexametaphosphate.
 4. The hollowmicrosphere of claim 1 wherein the multivalent metal is calcium,magnesium, aluminum or zinc.
 5. Water resistant hollow microspheres thathave not been exposed to fusion or sintering temperatures, saidmicrospheres being characterized by a weight % ratio of ammoniumpentaborate, sodium pentaborate or sodium hexametaphosphate solids tosodium silicate solids of 0.02:1 to 2.0:1, said sodium silicatecontaining 1.5 to 4.0 moles of SiO₂ per mole of Na₂ O; a particle sizeof about 35 to 2000 microns; a bulk density of 2 to 20 lbs/cu ft; lessthan 7% moisture and 0.1 to 5.0% of a multivalent metal that decreasesthe solubility of the microspheres in water.